Tin-doped indium oxide (indium tin oxide, hereinafter it may sometimes be referred to as “ITO”) is known as a material for forming a transparent conductive film used for transparent electrodes for a display element such as a liquid crystal display, an electroluminescent display or a plasma display; transparent electrodes for a touch panel or a solar cell; or a functional coating for heat reflection, electromagnetic wave-shielding material, antistatic material, or anti-fogging material.
A physical method such as a vacuum deposition method, a sputtering method or a chemical vapor deposition method has been widely used as a method for producing an ITO transparent conductive film. According to these methods, a uniform ITO transparent conductive film having both high transparency and high conductivity can be formed on a substrate.
These methods, however, are very expensive, because a vacuum container-based apparatus is used as a film-forming apparatus in these methods, and pressures of gas components in the production apparatus must be precisely controlled for each film formed on a substrate. Therefore, these methods are problematic in terms of production cost and mass productivity.
As a production method for solving the problems, a method wherein a substrate is coated with a coating liquid for forming a transparent conductive film in which an indium compound and a tin compound are dissolved in a solvent (hereinafter may sometimes be referred to as a “coating method”) has been studied.
According to this coating method, an ITO transparent conductive film is formed through simple production steps of coating a substrate with a coating liquid for forming a transparent conductive film, drying it, and baking it. As the coating method for coating a substrate with a coating liquid, an ink jet printing method, a screen printing method, a gravure printing method, an offset printing method, a flexo printing method, a dispenser printing method, a slit coating method, a die coating method, a doctor blade coating method, a wire bar coating method, a spin coating method, dip coating method, a spray coating method, and the like are known.
As a coating liquid used in the coating methods, various coating liquids containing an indium compound and a tin compound have been developed, and for example, a mixed liquid of an indium nitrate containing a halogen ion or a carboxyl group, and an alkyl tin nitrate (see, for example, JP-A-57-138708); a mixture of an organic indium compound containing an alkoxyl group or the like, and an organic tin compound (see, for example, JP-A-61-26679); a mixture of indium nitrate and an organic tin compound (see, for example, JP-A-4-255768); a mixture of inorganic compounds of indium nitrate, tin nitrate and the like (see, for example, JP-A-57-36714); a mixture of an organic indium nitrate such as an indium nitrate dicarboxylate and an organic tin nitrate such as an alkyl tin nitrate (see, for example, JP-A-57-212268); a mixed solution of organic compounds containing an organic indium complex in which acetylacetone is coordinated, and a tin complex (see, for example, JP-B-63-25448, JP-B-2-20706 and JP-B-63-19046), have been disclosed.
Many of these coating liquids which have hitherto been known make use of indium or tin nitrate, an organic or inorganic compound composed of a halide, an organic metal compound such as a metal alkoxide, or the like.
The coating liquid using the nitrate or halide, however, has a problem in which corrosion of equipment and deterioration of environment occur, because a corruptive gas such as nitrogen oxides or chlorine is generated at the time of baking; and the coating liquid using the metal alkoxide has a problem in terms of stability of the coating liquid, because its starting materials are easily hydrolyzed. In addition, many of the coating liquids using the organic metal compound described in Patent Documents, problematically, have poor wettability to a substrate, and likely provide an ununiform film.
As a coating liquid which improves these problems, a coating liquid for forming a transparent conductive film, containing acetylacetone indium (a full name: tris(acetylacetonato)indium: In(C5H7O2)3), acetylacetone tin (a full name: di-n-butyl bis(2,4-pentanedionato)tin: [Sn(C4H9)2(C5H7O2)2]), hydroxypropyl cellulose, an alkyl phenol and/or an alkenyl phenol, and a dibasic acid ester and/or benzyl acetate (see, for example, JP-A-6-203658) is disclosed.
The wettability of this coating liquid to a substrate is improved by containing the hydroxypropyl cellulose in the mixed solution of acetylacetone indium and acetylacetone tin, and, at the same time, the viscosity of the coating liquid is controlled by regulating the content of hydroxypropyl cellulose, which is a viscosity agent, whereby it becomes possible for the coating liquid to be applied to various coating methods such as spin coating, spray coating, dip coating, screen printing and wire bar coating.
As an improved coating liquid for spin coating, a coating liquid for forming a transparent conductive film, containing an organic indium compound such as acetylacetone indium or indium octoate, an organic tin such as acetylacetone tin or tin octoate and an organic solvent, wherein, as the organic solvent, an acetylacetone solution in which an alkyl phenol and/or an alkenyl phenol is dissolved, or an acetylacetone solution in which an alkyl phenol and/or an alkenyl phenol is dissolved, the solution being diluted with an alcohol, is used (see, for example, JP-A-6-325637), is proposed.
This coating liquid has a low viscosity, and in addition to spin coating, can be also used in spray coating or dip coating.